As a conventional technique, there is a technique for protecting copyright of digital content by embedding digital watermark into the digital content. In addition, there is a technique to refer to meta data such as copyright information on digital content. Further, there is a technique for taking a photograph of digital content, using a digital camera, via analog medium such as a printed matter as an advertisement so as to obtain information related to the advertisement by reading digital watermark.
As a method for embedding digital watermark into a still image, a digital watermarking scheme of spectrum spreading type is disclosed, the scheme being for embedding an embedding sequence that is generated using a pseudo-random number into a real part and an imaginary part of an orthogonal transformation region (Fourier transform region, for example) of the image, and performing detection using correlation between the embedding sequence and a detection sequence (refer to patent document 1, for example).
Also as to a video signal, generally, since the video signal is recorded as a sequence of frame images each being a still image, embedding of digital watermark becomes available by applying the digital watermark scheme for the still image. For example, digital watermark for the video signal can be realized by embedding digital watermark common to each frame image of the video signal by using the digital watermark method described in the patent document 1.
When the still image in which digital watermark is embedded is illegally used, it can be considered that a part of an image is cropped and it is used. In the case of the part of the image that is cropped, it cannot be ascertained which part of the original image corresponds to the cropped part in digital watermark detection that does not use the original image. This means that the embedded digital watermark pattern appears to be translated by an arbitrary amount. That is, this means a state in which the digital watermark pattern is desynchronized in a space direction. This is called “spatial synchronization of digital watermark”, and it is necessary to maintain spatial synchronization using a method for clarifying the translation amount and the like for detecting the digital watermark (generally, although spatial synchronization of digital watermark may include correction of geometrical deformation such as affine transformation, the present invention is targeted for correction of translation).
Generally, the video signal is treated as a set of a plurality of still images (frames) continuing in a time direction. In a digital watermark scheme for moving images, it is desired to be able to detect digital watermark from a set of a part of continuing frames in the set of the frames. For example, even when only one scene is cut away from distributed video content and it is used invalidly, by being able to detect digital watermark only from the invalidly used scene, effect of invalid use suppression can be expected. In addition, for example, in the case when detecting digital watermark from a re-taken video that is obtained by taking video content being projected in a movie theater using a video camera, a start point of the video when embedding digital watermark and a start point of the re-taken video are shifted inevitably. It is desired to be able to detect digital watermark also in such a case. In addition, for example, an application can be considered for detecting digital watermark from a video that is obtained by taking a scene currently being displayed in video content using a camera of a portable terminal and the like so as to obtain related information. In these examples, since it cannot be ascertained beforehand which part in the video in which digital watermark is embedded is cut away, it is necessary, when performing digital watermark detection, to know a subject part for detection corresponds to which position in the signal embedded as digital watermark. This is called temporal synchronization of digital watermark.
The spatial synchronization and the temporal synchronization in the conventional digital watermark scheme are broadly classified as follows.
(1) Exhaustive search: exhaustively trying detection of digital watermark successively for each of all amounts of desynchronization that can be considered;
(2) Embedding a signal for synchronization: embedding a signal for synchronization separately from digital watermark and detecting it for synchronization.
For example, in the digital watermarking method described in the patent document 1, a signal for detecting a spatial translation amount of digital watermark is embedded together with embedding information, and exhaustive search for amounts of desynchronization of the signal is efficiently performed using discrete Fourier transform so as to perform spatial synchronization. [Patent document 1] Japanese Laid Open Patent Application 2003-219148